Generally, as wiring boards for electronic devices, those that are produced by tightly adhering a prepreg (a resin insulating layer in a semi-cured state), which is obtained by impregnating a resin such as epoxy into a base material composed of glass fibers, aramid fibers or the like, to a metal foil of copper or the like and then forming a circuit thereon by an etching method. On such wiring boards, a solder resist is arranged for the purpose of inhibiting bleeding of solder at the time of component mounting. Wiring boards are desired to have high strength (high elastic modulus) for improvement of mountability, and their constituent members, which are prepregs and solder resists, are also intended to have a high elastic modulus in the same manner. Further, flexible wiring boards are used in electronic devices, and such flexible wiring boards are demanded to be multilayered in accordance with the need for densification and thickness reduction. Bonding sheets and cover lays are used as multilayering materials of conventional flexible wiring boards, and flexural resistance is demanded in wiring boards.
As prior art relating to a wiring board Material, for example, Patent Document 1 describes a prepreg in which, in order to attain both functions of insulation reliability and relaxation of the stress with wiring, resin layers having different strengths (elastic moduli) are arranged on each side of a core layer. In addition, Patent Document 2 describes a prepreg having a prescribed elastic modulus. This technology is aimed at temporarily fixing a prepreg sheet with a circuit board and controls the elastic modulus using a composition.
Patent Document 3 discloses a technology of using two resin compositions in a prepreg. In this prepreg, the two resin compositions are unevenly distributed in such a manner that the elastic modulus increases toward the surface side. Further, Patent Document 4 discloses a technology of arranging an adhesive layer on the surface of a prepreg for the purpose of improving the adhesion between the prepreg and a metal foil. Moreover, Patent Document 5 discloses a wiring board comprising a thin glass plate and a composite material of cellulose nanofibers and a resin.